As shown in FIG. 1, orbiting fluid particles describe elliptical paths in a wave W. The velocity vectors of the fluid particles describing the elliptical paths are directed in the moving direction of the wave at crests of the wave, directed downward midway between the crest and the bottom of the wave, directed opposite to the moving direction of the wave at the bottom of the wave, and directed upward midway between the bottom and the next crest of the wave. Non-patent document No. 1 discloses fundamental research results about rotation of a drag-type turbine provided with a plurality of buckets and located in a wave caused by fluid particles orbiting in the wave.